4,506 research outputs found
Polarisation dependence of magnetic Bragg scattering in YMnO
The polarisation dependence of the intensity of elastic magnetic scattering
from \ymno\ single crystals has been measured at 25 K in magnetic fields
between 1 and 9 T. A significant polarisation dependence was observed in the
intensities of magnetic satellite reflections, propagation vector
\pv=0.5,0,0.25 measured with both the [100] and [010] axes parallel to the
common polarisation and applied field direction. The intensity asymmetries
observed in sets of orthorhombicly equivalent reflections show systematic
relationships which allow the phase relationship between different components
of their magnetic interaction vectors to be determined. They fix the
orientation relationships between the small and moments on the \mnfp\
and \mntp\ sub-lattices and lend support to the structure reported by Kim et
al. It was found that that which suggests
that there is a small modulation of the nuclear structure which has the same
wave-vector as the magnetic modulation leading to a small nuclear structure
factor for the satellite reflections. The differences observed indicate shifts in the atomic positions of order 0.005 \AA
Cooperative Dynamics in Unentangled Polymer Fluids
We present a Generalized Langevin Equation for the dynamics of interacting
semiflexible polymer chains, undergoing slow cooperative dynamics. The
calculated Gaussian intermolecular center-of-mass and monomer potentials, wich
enter the GLE, are in quantitative agreement with computer simulation data. The
experimentally observed, short-time subdiffusive regime of the polymer
mean-square displacements, emerges here from the competition between the
intramolecular and the intermolecular mean-force potentials.Comment: 9 pages, latex, 3 figure
Inter-molecular structure factors of macromolecules in solution: integral equation results
The inter-molecular structure of semidilute polymer solutions is studied
theoretically. The low density limit of a generalized Ornstein-Zernicke
integral equation approach to polymeric liquids is considered. Scaling laws for
the dilute-to-semidilute crossover of random phase (RPA) like structure are
derived for the inter-molecular structure factor on large distances when
inter-molecular excluded volume is incorporated at the microscopic level. This
leads to a non-linear equation for the excluded volume interaction parameter.
For macromolecular size-mass scaling exponents, , above a
spatial-dimension dependent value, , mean field like density scaling
is recovered, but for the density scaling becomes non-trivial in
agreement with field theoretic results and justifying phenomenological
extensions of RPA. The structure of the polymer mesh in semidilute solutions is
discussed in detail and comparisons with large scale Monte Carlo simulations
are added. Finally a new possibility to determine the correction to scaling
exponent is suggested.Comment: 11 pages, 5 figures; to be published in Phys. Rev. E (1999
Surface topology assisted alignment of Min protein waves
Self-organization of proteins into large-scale structures is of pivotal importance for the organization of cells. The Min protein system of the bacterium Escherichia coil is a prime example of how pattern formation occurs via reaction-diffusion. We have previously demonstrated how Min protein patterns are influenced by compartment geometry. Here we probe the influence of membrane surface topology, as an additional regulatory element. Using microstructured membrane-clad soft polymer substrates, Min protein patterns can be aligned. We demonstrate that Min pattern alignment starts early during pattern formation and show that macroscopic millimeter-sized areas of protein patterns of well-defined orientation can be generated. (C) 2014 The Authors. Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies
Artificial Brains and Hybrid Minds
The paper develops two related thought experiments exploring variations on an ‘animat’ theme. Animats are hybrid devices with both artificial and biological components. Traditionally, ‘components’ have been construed in concrete terms, as physical parts or constituent material structures. Many fascinating issues arise within this context of hybrid physical organization. However, within the context of functional/computational theories of mentality, demarcations based purely on material structure are unduly narrow. It is abstract functional structure which does the key work in characterizing the respective ‘components’ of thinking systems, while the ‘stuff’ of material implementation is of secondary importance. Thus the paper extends the received animat paradigm, and investigates some intriguing consequences of expanding the conception of bio-machine hybrids to include abstract functional and semantic structure. In particular, the thought experiments consider cases of mind-machine merger where there is no physical Brain-Machine Interface: indeed, the material human body and brain have been removed from the picture altogether. The first experiment illustrates some intrinsic theoretical difficulties in attempting to replicate the human mind in an alternative material medium, while the second reveals some deep conceptual problems in attempting to create a form of truly Artificial General Intelligence
Structure of Colloid-Polymer Suspensions
We discuss structural correlations in mixtures of free polymer and colloidal
particles based on a microscopic, 2-component liquid state integral equation
theory. Whereas in the case of polymers much smaller than the spherical
particles the relevant polymer degree of freedom is the center of mass, for
polymers larger than the (nano-) particles conformational rearrangements need
to be considered. They have the important consequence that the polymer
depletion layer exhibits two widely different length scales, one of the order
of the particle radius, the other of the order of the polymer radius or the
polymer density screening length in dilute or semidilute concentrations,
respectively. Their consequences on phase stability and structural correlations
are discussed extensively.Comment: 37 pages, 17 figures; topical feature articl
Computation in Physical Systems: A Normative Mapping Account
The relationship between abstract formal procedures and the activities of actual physical systems has proved to be surprisingly subtle and controversial, and there are a number of competing accounts of when a physical system can be properly said to implement a mathematical formalism and hence perform a computation. I defend an account wherein computational descriptions of physical systems are high-level normative interpretations motivated by our pragmatic concerns. Furthermore, the criteria of utility and success vary according to our diverse purposes and pragmatic goals. Hence there is no independent or uniform fact to the matter, and I advance the ‘anti-realist’ conclusion that computational descriptions of physical systems are not founded upon deep ontological distinctions, but rather upon interest-relative human conventions. Hence physical computation is a ‘conventional’ rather than a ‘natural’ kind
An integral equation approach to effective interactions between polymers in solution
We use the thread model for linear chains of interacting monomers, and the
``polymer reference interaction site model'' (PRISM) formalism to determine the
monomer-monomer pair correlation function for dilute and
semi-dilute polymer solutions, over a range of temperatures from very high
(where the chains behave as self-avoiding walks) to below the
temperature, where phase separation sets in. An inversion procedure, based on
the HNC integral equation, is used to extract the effective pair potential
between ``average'' monomers on different chains. An accurate relation between
, [the pair correlation function between the polymer
centers of mass (c.m.)], and the intramolecular form factors is then used to
determine , and subsequently extract the effective c.m.-c.m. pair
potential by a similar inversion procedure. depends on
temperature and polymer concentration, and the predicted variations are in
reasonable agreement with recent simulation data, except at very high
temperatures, and below the temperature.Comment: 13 pages, 13 figures, revtex ; revised versio
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